PARASITISM 135 



inappreciable amount of material wliicli it might use itself, or feed upon 

 the waste products. Others are regarded as symbionts, which, living in 

 similar circumstances, not only derive benefit themselves, but contribute 

 to the well-being of the host. Thus, Cleveland (1923) has shown that 

 termites, which feed upon wood, do so by virtue of their intestinal Protozoal 

 fauna, which actually digest the wood to form substances on which the 

 life of the termites depends. Other forms are parasites, which deprive 

 their hosts of their own fluids or tissues, and damage them by destruction 

 of tissues either directly or indirectly through the formation of toxins. 

 The line of demarcation between these various types is very indefinite, 

 so that it is often impossible to decide to which group any particvdar 

 organism belongs. The numerous discussions which have arisen as to 

 the pathogenicity of the intestinal flagellates of man is a case in point. 



When true parasitism is considered, it must be remembered that the 

 degree of harm inflicted on the host has a direct bearing on the continued 

 existence of the parasite. A parasite is an organism which has become 

 adapted to an existence in another, and has lost at the same time the 

 power of living outside this host. At some period of its existence it must 

 be transferred to a new host if it is to survive. This transference may 

 take place by the production of encysted forms which escape from the 

 body and are taken up casually by a new host, or an invertebrate 

 may take up the parasites from the blood and later introduce 

 them to new hosts. In the first case the parasite does not appear 

 to be able to produce the encysted stages till some time after infec- 

 tion of a new host has taken place, and in the second a period must 

 elapse before the appearance in the blood of the forms capable of infecting 

 the invertebrate. In any case, the chance of a parasite gaining access to 

 a new host is a precarious one, and it is evident that the longer a parasite 

 can survive in one host, the better is its chance of bringing about infection 

 of another. If, then, a parasite is so virulent that it very quickly destroys 

 its host, its chances of continued existence are definitely diminished. It 

 is found in nature that there is such an adaptation of parasite to host, 

 and vice versa that in all cases of parasitism the parasite damages its host 

 to the least extent compatible with its own continued existence. When- 

 ever a parasite is discovered which brings about the death of its host in 

 a short time, it may safely be assumed that the host is not the natural 

 one, or that it is a natural one which is in some unnatural condition. In 

 the case of the pathogenic trypanosomes of Africa, the natural hosts are 

 the antelopes, to which they do comparatively little harm, while human 

 beings and domestic animals are unnatural hosts, as they are much more 

 seriously affected. After a time adaptation may occur, and a host which 

 was at first an unnatural one may gradually become a natural host. 



